1. Field of the Invention
The present invention relates to a pressure controlling method; at the time of heat sterilizing under pressure, a package formed from a flexible material including plastics, paper, aluminum foil and the like, and more particularly to one which is useful for application to an air-containing package in which a void exists between the package and its contents, or, among others, to a package with a relatively high proportion of air present therein (void volume).
2. Prior Art
Conventionally, at the time of subjecting foodstuffs and the like contained in a flexible package, such as a retort pouch, to heat sterilizing treatment under pressure in a sterilization treatment tank and the like, various pressure-controlling operations are effected from the viewpoint of preventing the breakage or deformation of packages which would result from pressure differential between the pressure in the treatment tank and that in the package.
For instance, a so-called high pressure-type sterilizing method is known wherein the pressure in the treatment tank is increased rapidly from the beginning of the sterilization treatment, constant increased pressure is maintained during a predetermined sterilizing time, and the pressure is decreased rapidly after cooling treatment (refer to the line C in FIG. 1). If this method is used for sterilizing an air-containing package, however, the pressure differential between the pressure in the treatment tank and that in the package at the time of temperature rise becomes extremely large, with the result that deformation and breakage would occur in the case of an ordinary thin-walled package.
In addition, a so-called constant pressure differential-type sterilizing method is also known whereby pressure is controlled while applying a constant pressure differential to saturated steam pressure in response to the temperature in the sterilization treatment tank (refer to the line D in FIG. 1). Even in this method, however, if the air-containing volume of a package which is subjected to sterilization treatment is high (such as when the proportion exceeds 20%), there is a delay before the change in the pressure in the package occurs following the change in the pressure in the treatment tank. Consequently, this results in the deformation of the package at the time of a temperature rise.
Furthermore, according to the aforementioned method, the timing of lowering the pressure in the treatment tank is set at the time of lowering the temperature in the treatment tank (the start of cooling). Accordingly, at the time when the interior of the treatment tank is cooled, the pressure in the package undesirably exceeds that in the treatment tank, resulting in the breakage of the package. Thus, it can be said that this method is not suitable to sterilization of a package with a high proportion of air present therein.
As another conventional method, a so-called dummy method is known wherein a small pot with a peep window is provided separately apart from the sterilization treatment tank, and the pressure in the tank is controlled on the basis of the state of deformation of a model package in the small pot.
According to the aforementioned method, however, it is necessary to provide the small pot, and a dummy package must be used every time sterilization is carried out, which is troublesome in terms of operation. Moreover, expert skill is required for visually discerning the state of deformation of the dummy package which serves as the basis of pressure control. Hence, it is extremely difficult to effect pressure control as desired.